When an aluminum pigment is incorporated into a water-based varnish, it has been conventionally known that problems exist such that the aluminum pigment reacts with water contained in the varnish, hydrogen gas is generated, and a container explodes during storage, that the aluminum pigment hardens and generates a granular structure, etc. Many techniques have been developed to solve this problem.
A method for treating an aluminum pigment with chromic acid is disclosed in Japanese Patent Publication No. 01-054386 (National Patent Publication No. 01-501631, Patent Document 1). According to this method, the chemical stability of the aluminum pigment is improved. However, it is not much put to practical use due to difficult points that the aluminum pigment having a small particle diameter cannot be treated because reactivity of the treatment liquid is too strong, that a problem from the viewpoint of industrial health or the environment is large because a hexavalent chromium compound is used, etc.
A method for treating an aluminum pigment with a treatment liquid containing an oxidizer such as molybdic acid, a phosphate ion, and an alkaline-earth metallic ion is disclosed in Japanese Patent Laying-Open No. 04-318181 (Patent Document 2). According to this method, the chemical stability of the aluminum pigment is improved. However, there is a tendency that the phosphate ion or the alkaline-earth metallic ion contained in the treatment liquid decrease moisture resistance and other physical properties of a coating film.
A method for treating an aluminum pigment with hetero polyanion such as phosphomolybdic acid is disclosed in U.S. Pat. No. 5,296,032 (Patent Document 3). However, sufficient chemical stability cannot be imparted to the aluminum pigment with this method, and a phosphate ion contained in the treatment agent decreases physical properties such as moisture resistance of the coating film.
A method for further stabilizing an aluminum pigment by treating the aluminum pigment with ammonium molybdate and then adding molybdate, etc. is disclosed in Japanese Patent Laying-Open No. 06-057171 (Patent Document 4). An aluminum pigment having relatively good chemical stability can be obtained with this method. However, the chemical stability is not necessarily sufficient, and there is a problem that the production process is complicated.
An aluminum pigment treated by peroxypolyacid is disclosed in Japanese Patent Laying-Open No. 09-328629 (Patent Document 5). Because a film derived from peroxypolyacid is dense and is excellent in corrosion resistance, an aluminum pigment can be obtained that is chemically stable against a water-based paint and a varnish for a water-based ink by forming this film on the aluminum pigment surface. As described above, this aluminum pigment has excellent chemical stability. However, because dispersibility of the aluminum pigment is not very good, there are problems that a granular structure is generated when forming a coating film and that appearance becomes poor.
Based on such conditions described above, the present inventors have found and proposed that an aluminum pigment having a film derived from peroxypolymolybdic acid and containing amine, and further an aluminum pigment having an adsorption layer of an organic phosphorus compound formed on the film, exhibit excellent water resistance (International Publication No. WO 2002/031061 (Patent Document 6)).
The aluminum pigment that is excellent in water resistance described above is a pasty aluminum pigment produced by forming an inorganic film on the aluminum surface by adding amine, a solution in which metallic molybdenum is dissolved in a hydrogen peroxide solution, and a hydrophilic solvent to an aluminum pigment composition containing an organic solvent, stirring and mixing in a heated condition, and by adding an organic phosphorus compound depending on necessity. The aluminum pigment is used as a metallic pigment by being incorporated upon the production of a paint that is the same as the conventional pasty aluminum pigment. This pasty water-resistive aluminum pigment can be used broadly because it is excellent particularly in water resistance and hardly aggregates and the characteristics of the coating incorporating and using this are also excellent. However, there has been a problem that the process is difficult to be controlled because heating is necessary in stirring and mixing for forming the film derived from peroxypolymolybdic acid on the aluminum surface, amine is necessary to suppress the aggregation due to an intense reaction of peroxypolymolybdic acid with aluminum, etc.
Further, a method for solving the above-described problem is disclosed in Japanese Patent Laying-Open No. 2003-301131 (Patent Document 7) in which similar water resistance can be obtained by incorporating an aluminum pigment dispersion of which peroxypolymolybdic acid, the aluminum pigment, water, and a dispersing agent are simply stirred and mixed when making it into a paint without using the aluminum paste previously formed on the aluminum surface by heating the film derived from peroxypolymolybdic acid. However, because peroxypolymolybdic acid is a strong acid solution in this method, a resin can be made unstable depending on the types and composition of the paint resin, and there has been a problem that the aluminum pigment aggregates in the paint.    Patent Document 1: Japanese Patent Publication No. 01-054386 (National Patent Publication No. 01-501631)    Patent Document 2: Japanese Patent Laying-Open No. 04-318181    Patent Document 3: U.S. Pat. No. 5,296,032    Patent Document 4: Japanese Patent Laying-Open No. 06-057171    Patent Document 5: Japanese Patent Laying-Open No. 09-328629    Patent Document 6: International Publication No. WO 2002/031061    Patent Document 7: Japanese Patent Laying-Open No. 2003-301131